if their denominators are the same, just add it or subtract. but if they don't have the same denominators, you have to find their LCD ( Least Common Denominator )
example in the same denominators:
2/4 + 1/4 = 3/4
in different denominators:
2/3 - 2/4= 8/12 - 6/12 = 2/12 or 1/6
